(R)-3-N,N-Dicyclobutylamino-8-fluoro-3,4-dihydro-2H-1-benzopyran-5-carboxam ide is disclosed in WO 95/11891 as is a process for preparing the named compound. Said process comprises a number of reaction steps. The fluorine atom is introduced into the benzopyran nucleus by selective bromination in the 8-position, followed by N,N-dibenzylation, followed by halogen-lithium exchange of the bromo compound and reaction with a suitable fluorinating agent. The obtained (R)-3-N,N-dibenzylamino-8-fluoro-5-methoxy-3,4-dihydro-2H-1-benzopyran is then subjected to debenzylation, N,N-dialkylation by reductive alkylation with cyclobutanone; demethylation; and catalytic conversion using a transition metal, carbon monoxide and an appropriate alcohol, resulting in the production of the intermediate alkyl (R)-3-N,N-dicyclobutylamino-8-fluoro-3,4-dihydro-2H-1-benzopyran-5-carboxy late. Hydrolysis of the ester to the carboxylic acid, followed by treatment of the acid with thionyl chloride gives the acid chloride which upon treatment with ammonia gives the desired (R)-3-N,N-dicyclobutylamino-8-fluoro-3,4-dihydro-2H-1-benzopyran-5-carboxa mide.
The above notwithstanding there is still a need for new, more convenient and efficient processes of manufacturing (R)-3-N,N-dicyclobutylamino-8-fluoro-3,4-dihydro-2H-1-benzopyran-5-carboxa mide.
The process according to the present invention for the preparation of 3-N,N-dicyclobutylamino-8-fluoro-3,4-dihydro-2H-1-benzopyran-5-carboxamide is more advantageous from a technical point of view than the process described in WO 95/11891. The claimed process employs a starting material which has the fluoro-substituent in place and thus does not require the undesirable fluorination step. The later introduction of fluorine according to the conventional process requires low temperature lithiation and reaction with an expensive, hazardous and possibly toxic fluorinating agent. Furthermore, this reaction yields a substantial amount of (R)-3-N,N-dibenzylamino-5-methoxy-3,4-dihydro-2H-1-benzopyran as a by-product which must be separated from the desired fluorinated end-product by a costly and technically difficult chromatography method. The process according to the present invention is therefore commercially more advantageous than the process known from WO 95/11891.